Finally the secrets behind the McDonald's best-selling Shamrock Shake have been revealed. Strangely, the bright green minty treat dates back to the 1970, and its story is based on charity.

The Shamrock Shake was developed in 1970, according to McDonald's. However, according to reports on Delish.com, Harold Rosen, who owned a McDonald's in Connecticut, claims he invented the popular holiday drink in 1966.

The McDonalds website says, "It all began with a little girl, a football team, and a visionary doctor."

The Philadelphia Eagles tight end Fred Hill's daughter was diagnosed with leukemia and began treatment. The Hill family were camping out in waiting rooms in hospitals and saw other families doing the same. They realized that many families travelling long distances to visit their sick family members had nowhere to stay and could not afford hotel rooms. The Hill family did something about it.

Hill and his team decided to raise some funds. The team's general manager, Jim Murray, called a friend in McDonald's advertising and suggested the team on the next promotional push. It just happened to be St. Patrick's Day.

The Shamrock Shake raised enough money to buy a four-story house which was the first Ronald McDonald House Charity.

Imagine -- the Shamrock Shake now has 21,500 fans on Facebook and is second in popularity only to the McRib, and that sandwich had a "Simpsons" episode dedicated to it!